As a related art developing device, for example, JP-A-10-240008 describes a process cartridge, in which a development roller and a developer cartridge are housed in a casing, and the developer cartridge is detachable with respect to the casing.
The casing includes a drive shaft member for receiving a driving force from a motor or the like. An agitation part for agitating developer in the interior of the developer cartridge is accommodated in the developer cartridge, and an engaging recess is provided in an end portion of a rotating shaft of the agitation part. When the developer cartridge is attached to the casing, the drive shaft member is fitted in and coupled with the engaging recess, and the driving force is transmitted from the drive shaft member to the agitation part.
In addition, a developer sensor is provided on the casing. The developer sensor is provided with a light emitting portion and a light receiving portion. The light emitting portion is opposed to the light receiving portion with the developer cartridge placed therebetween. A detection light emitted by the light emitting portion is passed through developer in the developer cartridge and is received by the light receiving portion. Since the light reception frequency of the detection light at the light receiving portion differs in correspondence with the amount of developer in the developer cartridge, the amount of developer in the developer cartridge is detected in correspondence with the light reception frequency of the detection light.